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    SubjectRe: [patch 13/14] x86_64: Use common functions in cluster and physflat mode
    On Mon, Sep 12, 2005 at 01:02:20AM +0200, Andi Kleen wrote:
    > On Sun, Sep 11, 2005 at 09:44:16AM -0700, Zwane Mwaikambo wrote:
    > > On Fri, 9 Sep 2005, Ashok Raj wrote:
    > >
    > > > IPI performance worse, since it would do one cpu at a time, and requires 2
    > > > writes per cpu for each IPI v.s just 2 for a flat mode mask version of the API.
    > >
    > > I don't see the benefit then :/ I certainly hope we don't go that route.
    >
    > I originally made that objection, but Ashok then did some benchmarks
    > that showed essentially no difference. I can see the point - it's


    Just a minor difference is that for a 8 CPU system

    Using IPI shortcut uses just 1 write to local apic
    Using mask version of broadcast uses 2 writes to local apic

    The stat showed no significant difference when we do the 1 extra write. But
    that is for the whole system.

    When we use send_IPI_mask_sequence() we use 14 writes to (two writes
    for each CPU that we need to target a IPI).

    Using the same stats i used earlier, i see about 0x200 - 0x1000 extra cycles
    when using mask_sequence() on certain long runs about 100K samples.

    We should probably remove the !HOTPLUG case and just use the mask version
    for all cases <=8 CPUS, use physflat or the cluster mode for >8cpus as
    the case may be, instead of defaulting to sequence_IPI which seems
    a little overkill for the intended purpose.

    > likely that an access to the local APIC (which is in the CPU) is fast
    > (we know it is) and all the time is dominated by sending the
    > requests over the wires between the CPUs. So it shouldn't matter
    > much if you use sequence mode or masks.
    >
    > That is why I changed my mind and just made physflat default for the hotplug
    > case (which will be essentially everywhere because I expect most kernels
    > to have hotplug enabled in the future)
    >
    > It's a bit of a mess in mm right now because me and Ashok have been fixing
    > similar problems in a different way (e.g. the patch in flat to
    > use the sequence sending path is also not needed anymore with that)
    > Need to clean this up a bit.
    >
    > Handling it properly for i386 is also still needed e.g. the older physflat
    > patch I did needs to be merged with bigsmp and cleaned up a bit.
    > But I don't have time to do it before monday so it'll miss the 2.6.14
    > window anyways.
    >
    > -Andi

    --
    Cheers,
    Ashok Raj
    - Open Source Technology Center
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